1. Field of the Invention
The present invention relates to a television tuner, particularly relates to a television tuner that suppresses the outside radiation of an inside oscillated signal.
2. Description of the Related Art
FIG. 2 is a layout viewed from the top showing a conventional type television tuner. As shown in FIG. 2, a rectangular printed board 31 is housed in a metallic frame 32 that holds the four sides. A connector 33 to which a television signal is input is attached to one side face of the metallic frame 32. A high-frequency circuit 34, a first integrated circuit 35 and a second integrated circuit 36 are sequentially mounted on the printed board 31 from the side on which the connector 33 is attached toward the opposite side. The high-frequency circuit 34 is provided with a tuning circuit (not shown), a high frequency amplifying circuit 34a and others and is connected to the connector 33 to take a television signal. The high-frequency circuit selects a television signal to be received, amplifies and outputs it.
The first integrated circuit 35 is provided with four edges and inside it, a mixing circuit for converting an input television signal to an intermediate frequency signal, an intermediate frequency amplifying circuit that amplifies the intermediate frequency signal, an oscillation circuit that supplies a local oscillated signal to the mixing circuit, a PLL circuit that controls the frequency of the local oscillated signal, a reference oscillation circuit that supplies a reference signal to the PLL circuit and others (respectively not shown) are mounted. The first integrated circuit 35 is provided with terminals on two opposite edges and the terminals are mounted so that one edge is opposite to the high-frequency circuit 34 and the other edge is opposite to the second integrated circuit 36. An input terminal 35a of a television signal is provided to one edge and on the other edge, an intermediate frequency output terminal 35b for outputting an intermediate frequency signal, a power supply terminal 35c to which source voltage is supplied, an oscillator terminal 35d for connecting an oscillator provided outside (not shown) to the reference oscillation circuit and others are provided. The input terminal 35a is connected to the high-frequency circuit 34 via a conductive pattern 31a formed on the printed board 31.
Plural external terminals 37 are attached to the printed board 31 in the vicinity of the first side adjacent to the side on which the connector 33 is attached. The external terminals 37 are protruded from the metallic frame 32 outside and are connected to a mother board not shown. The intermediate frequency output terminal 35b of the first integrated circuit 35 is connected to the external terminal 37a for outputting an intermediate frequency signal via a conductive pattern 31b formed on the printed board 31, the power supply terminal 35c is connected to the external terminal 37b to which power is supplied via a conductive pattern 31c and the oscillator terminal 35d is connected to the external terminal 37c to which the oscillator is connected via a conductive pattern 31d. NF measurement equipment is connected to the external terminal 37a when a noise factor (NF) is measured. Source voltage is applied to the external terminal 37b. Source voltage is also supplied to the high-frequency circuit 34 via the conductive pattern 31c. 
The second integrated circuit 36 is also provided with four edges and inside, a demodulating circuit, an AGC voltage generating circuit and others (none of them shown) are mounted. Terminals are provided on two opposite edges. The second integrated circuit 36 is mounted so that edges on which terminals are provided are perpendicular to the edges on which the terminals are mounted of the first integrated circuit, that is, one edge is opposite to a second side on the reverse side to the first side of the printed board 31 and the other edge is opposite to the first side. On one edge, an intermediate frequency input terminal 36a is provided and on the other edge, a power supply terminal 36b and an AGC terminal 36c for taking AGC voltage are provided.
An SAW filter 38 is mounted in the vicinity of the first integrated circuit 35 and the second integrated circuit 36 on the printed board 31 and is connected between the intermediate frequency output terminal 35b of the first integrated circuit 35 and the intermediate frequency input terminal 36a of the second integrated circuit 36 via the conductive pattern 31b. The power supply terminal 36b is connected to the power supply external terminal 37b via the conductive pattern 31c. The AGC terminal 36c is connected to the AGC external terminal 37d via a conductive pattern 31e, is also connected to the high-frequency circuit 34 and AGC voltage is supplied to the high frequency amplifying circuit 34a. The AGC external terminal 37d is used for monitoring.
In the configuration described above, each conductive pattern 31b, 31c, 31d, 31e crosses each other in a layout. For example, the conductive pattern 31b connected to the intermediate frequency external terminal 37a crosses the conductive pattern 31c connected to the power supply external terminal 37b, the conductive pattern 31d connected to the oscillator external terminal 37c and the conductive pattern 31e connected to the AGC external terminal 37d. In such an intersection, an insulating film is provided between the conductive patterns to prevent mutual contact.
As the oscillation circuit is provided in the first integrated circuit, a signal oscillated there is superimposed on the conductive pattern 31c that supplies source voltage and the conductive pattern 31d connected to the oscillator external terminal 37c. The signal is also superimposed on the conductive pattern 31b for taking an intermediate frequency signal which crosses these conductive patterns via these conductive patterns 31c, 31d. The oscillated signal is radiated outside from the external terminals. To prevent the radiation, the power supply external terminal is directly grounded via a capacitor by a bypass. However, a capacitor for a bypass cannot be connected to the intermediate frequency external terminal 37a because the capacitor has a bad effect upon the characteristic of an intermediate frequency signal. Therefore, there is a problem that the superimposed oscillated signal is radiated outside via the intermediate frequency external terminal 37a and the product does not meet standards such as the FCC Standard in the United States.